Seal hardening furnace of liquid crystal display device having rack bar

ABSTRACT

A seal hardening furnace is presented in which seal lines in a liquid crystal display panel are hardened. The seal hardening furnace includes a cassette having a rack bar structure. The rack bar structure has rack bars for supporting the substrate along one direction and rack bar supports at ends of the rack bars that support the rack bars. The rack bars have air discharge openings therein. An air injecting passage is connected to the rack bar supports. An air supply unit supplies air through the air injecting passage and through the discharge openings to support the substrate thereon.

PRIORITY CLAIM

The present application claims the benefit of priority to KoreanApplication 2004-115561, filed on Dec. 29, 2004, which is hereinincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a seal hardening furnace of a liquidcrystal display device, and particularly, to a structure of a rack barwithin a seal hardening furnace.

DESCRIPTION OF THE RELATED ART

In general, a liquid crystal display device includes a liquid crystaldisplay panel having unit pixels formed thereon in a matrixconfiguration, a driving circuit unit for supplying a scanning signaland an image signal to the liquid crystal display panel, and aback-light unit for supplying light to the liquid crystal display panel.

In particular, the liquid crystal display panel is equipped with anupper color filter substrate, a lower array substrate, and a liquidcrystal layer filled in a position between the two substrates. The twosubstrates, namely, the color filter substrate and the array substrateare attached to each other by seal lines formed at their edges.

Hereinafter, a process for fabricating the liquid crystal display panelwill briefly be described. The process for fabricating the liquidcrystal display panel includes forming the array substrate on which athin film transistor used as a switching device is formed; forming thecolor filter substrate on which a color filter layer for renderinginformation with color; forming seal lines on one certain substrate ofthe two substrates; attaching the two substrates by the seal lines; andcutting the attached substrates into unit liquid crystal display panels.The liquid crystal layer may be formed after performing the cuttingprocess when adopting an injecting method. Otherwise, the process forforming the liquid crystal layer may be carried out at the same time asthe attaching process is ongoing when adopting a dispensing method.

In general, a plurality of liquid crystal display panels are formed on amother substrate so as to improve productivity. When a plurality ofarray substrates and color filter substrates are formed on one mothersubstrate, one of the two substrates is coated by a sealant which ismade of thermosetting resin to thereafter heat up for attaching an uppersubstrate (the color filter substrate) and a lower substrate (the arraysubstrate).

The process for attaching the upper substrate and the lower substrate ofthe liquid crystal display panel will now be explained in more detailwith reference to FIGS. 1A and 1B.

As shown in FIGS. 1A and 1B, the array substrate and the color filtersubstrate are substantially attached by heating them within a chamber10, using an attaching device.

When the array substrate and the color filter substrate are completelyformed, seal lines are formed at edges of one of the two substrates.Thereafter, as shown in FIG. 1A, an upper substrate 8 b and a lowersubstrate 8 a of the liquid crystal display panel are fixed,respectively, to a lower stage 2 and an upper stage 4 within the chamber10.

After fixing the two substrates, the upper stage 4 descends to therebyprovide a uniform pressure on the lower substrate 8 a. At this time, theupper substrate 8 b and the lower substrate 8 a are attached to eachother by the seal lines due to the provided pressure.

The attaching process is carried out within a chamber 10 which is heatedup to a certain temperature. However, after completing the attachingprocess, the substrate undergoes a hardening process. While hardeningit, a pressure in an inner space of the liquid crystal display panelwhich is formed by the attachment is different from an air pressure ofthe hardening chamber. As a result, the pressure acts upon the seallines so as to make holes in the seal lines.

Therefore, in order to temporarily harden the seal lines during theattaching process, a UV (Ultra-Violet) hardening process for irradiatingthe UV radiation on the seal lines is carried out. Hence, the chamber inwhich the attaching process is performed is further provided with a UVlamp for generating the UV radiation.

Accordingly, after completely carrying out the attaching process withinthe chamber 10, a primary hardening for the seal lines by the UVradiation is performed.

Afterwards, a secondary hardening for the liquid crystal display panelfor which the primary hardening has been performed is carried out in aseal hardening furnace, thereby completing the attaching process. Thesecond seal hardening is performed in a separate chamber from thechamber in which the primary hardening is performed.

The secondary hardening is substantially carried out within the chamberfor a predetermined time in a heated state. The chamber has a cassettetherein which is equipped with a rack bar, so that the seal hardeningfor the plurality of substrates can be performed.

With reference to FIGS. 2A and 2B, structures of the cassette and therack bar within the chamber in which the seal hardening is carried outwill now be described. FIG. 2A is a perspective view illustrating acassette within a related art seal hardening furnace and FIG. 2B is afront view thereof.

With reference to FIGS. 2A and 2B, a cassette 200 within a sealhardening furnace (not shown) is composed of a plurality of layers forplacing a plurality of substrates 204 thereon. A plurality of rack bars201 for supporting the substrate (not shown) is installed on each layerin a Y axis direction thereof, and a rack bar support 202 for supportingthe rack bars 201 is installed on a front end of the rack bar 201 in anX axis direction on each layer. Therefore, the front end of the rack bar201 is positioned on an upper surface of the rack bar support 202, and arear end thereof is installed on a rear surface of the cassette 200 andfixed by a second rack bar support 205 which is installed in a Z axisdirection.

The rack bars 201 have certain heights and are installed with a certaininterval to thereby support the substrate loaded thereon. Accordingly,uniform spaces 203 are formed between the rack bars 201 adjacent to eachother, and fingers 206 of a robot arm are introduced into and drawn outthrough the spaces 203 to load and unload the substrate.

However, in order to harden the seal lines formed of thermosettingresin, the substrate is typically placed in the seal hardening furnacefor a long time in a high temperature state. At this time, stain isgenerated on the substrate region in contact with the rack bars 201,which causes defects in the liquid crystal display panel.

In particular, due to the large-size liquid crystal display panel beingheavy, a more substantial amount of strain may be generated in theregion where the substrate contacts the rack bars 201.

SUMMARY OF THE INVENTION

By way of introduction only, a seal hardening furnace is provided thatis capable of mitigating stain generation in a liquid crystal displaypanel during a hardening process when fabricating a liquid crystaldisplay device. More particularly, stain generation in a liquid crystaldisplay panel is mitigated while hardening seal lines by improving acassette structure in which the liquid crystal display panel is loadedin a seal hardening furnace.

In one embodiment, a seal hardening furnace comprises a cassetteincluding rack bars for supporting a substrate and having dischargeopenings therein, a first rack bar support for supporting the rack barsin an X axis direction, an injecting passage connected to the first rackbar support, and an air supply unit for supplying air through theinjecting passage.

According to another embodiment of the present invention, there isprovided a seal hardening furnace comprising first rack bars forsupporting the substrate, second rack bars connected to the first rackbars, a first rack bar support for supporting the first rack bars in anX axis direction, and a second rack bar driving unit installed at a rearend of each of the second rack bars.

In another embodiment, a seal hardening furnace comprises a cassetteincluding rack bars, a rack bar support disposed at an end of the rackbars that supports the rack bars, and a secondary support mechanism thatincreases uniformity of support for substrates disposed on the rack barsbeyond support provided by the rack bars alone; and a heating unit forheating the substrate. The secondary support mechanism comprises, forexample, a gas discharge mechanism that provides gaseous support for thesubstrate such that the substrate is disposed over the rack bars withoutcontacting the rack bars and/or a secondary rack bar movably connectedto each of the rack bars such that a contact area of a surface of therack bars and the substrate is extended when the secondary rack bar isin an extended position.

The foregoing and other features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIGS. 1A and 1B illustrate an inside of a chamber of an attaching deviceof a related art liquid crystal display panel;

FIGS. 2A and 2B are perspective view and plane view, respectively,showing a cassette structure within a seal hardening furnace;

FIGS. 3A and 3B are perspective view and plane view, respectively,showing a cassette structure within a seal hardening furnace inaccordance with a first embodiment of the present invention;

FIGS. 4A to 4D illustrate a rack bar structure and an operation thereofin accordance with a second embodiment of the present invention; and

FIG. 5 illustrates a rack bar structure equipped with a substrate fixingmember according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

With increasing sizes of liquid crystal display devices, the liquidcrystal display panel has increased in weight to be as heavy as severalkilograms. In particular, the liquid crystal display panel is formed byattaching an upper substrate and a lower substrate and forming a liquidcrystal layer therebetween. The two substrates are attached by a sealantformed of thermosetting resin. The attaching process using the sealantis typically carried out within a chamber. After performing theattaching process, a separate process for hardening the sealant iscarried out. The hardening process is performed within a heat hardeningfurnace. The liquid crystal display panel is hardened in a hightemperature state by being loaded on a cassette of the heat hardeningfurnace.

The hardening process is carried out for about a hour under atemperature of 100˜120° C. Because of the heavy weight of the liquidcrystal display panel, while undergoing the seal hardening, a columnspacer is pressed down at a region where rack bars of the cassette forsupporting the liquid crystal display panel are contact with the liquidcrystal display panel, thereby resulting in a screen defect such as astain on the liquid crystal display panel. The screen defect is causedby the column spacer being pressed down by a load of the liquid crystaldisplay panel during the seal hardening process.

In order to prevent the column spacer from being pressed by the load ofthe liquid crystal display panel although the seal hardening is carriedout for a long time, gas, such as air, is injected into a contactsurface between the liquid crystal display panel and the rack bar tothereby lift up the liquid crystal display panel from the rack bar, or acontact area between the liquid crystal display panel and the rack baris widened by changing the rack bar structure. As a result, the load ofthe liquid crystal display panel is more uniformly dispersed to therebyprevent the column spacer from being pressed.

Hereinafter, a seal hardening furnace for hardening a sealant of aliquid crystal display panel will be described with reference to FIGS.3A and 3B.

First, before explaining the seal hardening furnace, an attachingprocess for the liquid crystal display panel will now be explained.

When an array substrate and a color filter substrate which arefabricated by a separate process are delivered, seal lines formed ofthermosetting resin are formed on one of the two substrates.

For instance, the seal lines may be formed on four edges of the colorfilter substrate by a seal line dispenser. Afterwards, the color filtersubstrate and the array substrate having the seal lines thereon arealigned each other, and are heated and pressurized in a vacuum chamber,thereby being attached to each other. After attaching the two substratesto each other, a seal hardening for the liquid crystal display panel iscarried out in a separate chamber. However, since the seal hardening isperformed under an atmospheric pressure, the seal lines may be destroyedby an air pressure difference between the seal hardening chamber underthe atmospheric pressure circumstance and a vacuum space within theliquid crystal display panel formed by the seal lines. Hence, aftercompleting the attaching process, the seal lines are temporarilyhardened by UV (Ultra-Violet) radiation source within the chamber inwhich the attaching process is ongoing (it is referred to as a UV cure).Therefore, the sealant contains a UV hardener which is hardened by theUV radiation.

Once primarily hardening the seal lines by irradiating the UV radiation,the liquid crystal display panel, which has not been completely hardenedby the primary hardening, is transferred to a seal hardening furnacewhich is a separate chamber, and the non-hardened seal lines thereof arethen hardened.

The chamber in which the seal hardening is carried out maintains atemperature of about 120° C. and the seal lines are hardened in thechamber for about one hour. Moisture of the seal lines formed ofthermosetting resin is removed therefrom in a high temperature state fora long time so that the seal lines are hardened.

The seal hardening furnace is provided with a cassette 300 for storing aplurality of liquid crystal display panels for which the seal hardeningprocess is carried out. The cassette structure will now be explained.

With reference to FIGS. 3A and 3B which are a perspective view and aplane view, respectively, showing a substrate storing cassette within aseal hardening furnace according to a first embodiment of the presentinvention, the cassette 300 includes a plurality of frames 310 composinga cuboid cassette body, a first rack bar support 303 installed in an Xaxis direction, for connecting front frames of the cassette, a secondrack bar support 305 installed at a rear end of the cassette in a Z axisdirection, a plurality of rack bars 302 of which front ends arepositioned on the first rack bar support 303 and rear ends are fixed tothe second rack bar support 305, and a substrate 301 loaded on the rackbars 302 and for which a seal hardening process is to be carried out.

The rack bar 302 has a certain height and its upper surface has a flatbar shape. Because the rack bars 302 are installed with a uniforminterval on the first rack bar support 303, spaces are formed betweenthe rack bars 302 adjacent to each other. Fingers of a robot arm forloading and unloading the substrate 301 on the rack bars 302 are thuslyintroduced into and drawn out through the spaces.

A plurality of air discharge openings 340 is distributed in an uppersurface of each rack bar 302 in a uniform density. A passage forsupplying air is installed inside each rack bar 302 so as to eject airthrough the air discharge openings 340. Therefore, the inside of therack bar 302 may be formed in a shape of a cavity or a pipe connected tothe air discharge openings 340 may be formed in the rack bar 302.

Furthermore, the rack bar 302 and the first rack bar support 303 areconnected to each other. A pipe 350 for injecting air into the rack bar302 through the first rack bar support 303 is installed in the firstrack bar support 303. The pipe of the rack bar 302 and the pipe 350 ofthe first rack bar support 303 are connected to each other.

An air injecting passage 320 for supplying air to the first rack barsupport 303 is formed at a side surface of the first rack bar support303. An air generator 330 for supplying air is further installed at oneend of the air injecting passage 320.

The cassette 300 is configured with several layers to thereby heat up aplurality of substrates at the same time. The first rack bar support 303and the rack bars 302 are installed at each layer, and the substrate isloaded on an upper surface of the rack bars 302. Each first rack barsupport 303 is connected to the air injecting passage 320 for receivingair supplied thereto.

Because the seal hardening furnace is operated at a temperature of about100˜120° C., air supplied by the air generator 330 is heated up to apredetermined temperature. Therefore, the air generator 330 may furtherhave a heating unit (not shown) therein for heating the air.

An air outlet 360 for discharging the supplied air at a uniform speed isfurther installed at one end of the seal hardening furnace, therebyuniformly maintaining an air pressure in the seal hardening furnace.

It will now be described how to harden the seal lines on a substrate byusing the seal hardening furnace with such structure.

Seal lines of a liquid crystal display panel are primarily hardened byan attaching process within a chamber and a UV hardening process.

Afterwards, the liquid crystal display panel of which seal lines haveprimarily been hardened is transferred to a seal hardening furnaceaccording to the present invention for hardening the seal linescompletely. The liquid crystal display panel is transferred by a robotarm. Fingers of the robot arm are introduced into and drawn out throughspaces formed between the rack bars adjacent to each other to therebyload the substrate on the rack bars.

Next, the seal hardening furnace is gradually heated up to about 120° C.At this time, air generated by the air generator 330 passes through theair injecting passage 320, the first rack bar support 303, and the rackbars 302 in sequence, to thereafter be sprayed out through the airdischarge openings 340. The substrate 301 is then lifted from the rackbars 302 by the sprayed air by a certain height. A pressure of thesprayed air is optimized according to a weight of the substrate to beloaded. A height which the substrate 301 is lifted up may be less thanseveral millimeters, which is enough if the rack bar and the substrateare prevented from being contact therewith only while the seal hardeningprocess is carried out.

The seal hardening process is carried out for about a hour. Whileperforming the seal hardening, air is continuously injected inwardly.While this, the air outlet 360 installed at one side of the sealhardening furnace sprays air outwardly at a uniform speed to therebymaintain a pressure within the seal hardening furnace at a uniformlevel.

When all of the seal lines on the substrate 301 are completely hardened,the air generator 330 stops its operation and the substrate 301 isunloaded from the seal hardening furnace by the robot arm.

Therefore, once hardening all of the seal lines by using the sealhardening furnace with the aforementioned structure, while the seallines are hardened, the substrate 301 does not directly contact the rackbars 302, and accordingly it is possible to mitigate stain in the liquidcrystal panel caused by contact between the rack bar and the substrate301.

In the embodiment, the substrate 301 does not contact the rack barswhile the seal lines are hardened. In a second embodiment describedherebelow, while the seal lines are hardened, an area in which the rackbars 302 contact the substrate 301 is widened, and accordingly the loadof the substrate is uniformly dispersed to thereby prevent the stainfrom being generated.

A basic configuration of a cassette installed in the seal hardeningfurnace in accordance with the second embodiment of the presentinvention is similar to that of the first embodiment. That is, acassette according to the second embodiment includes a plurality offrames composing a cuboid cassette body, a first rack bar support 303installed in an X axis direction, for connecting frames positioned at afront surface of the cassette, a second rack bar support 305 installedat a rear end of the cassette in a Z axis direction, a plurality offirst rack bars 401 each of which front end is positioned on the firstrack bar support 303 and rear end is fixed to the second rack barsupport 305, a plurality of second rack bars 402 connected to the firstrack bars 401, a driving unit for folding or unfolding the second rackbars 402 from the first rack bars 401, and a substrate loaded on thefirst and second rack bars 401 and 402.

Moreover, the second rack bars 402 are connected to the first rack bars401, respectively, by a connection member 403 such as hinges. Thedriving unit is connected to a rear end of the second rack bar 402 tothereby fold or unfold the second rack bar 402 from the first rack bar401 by a horizontal motion.

Hereinafter, main parts of a rack bar structure in accordance with thesecond embodiment will be described with reference to FIGS. 4A and 4B.

As shown in FIG. 4A, a front end of each first rack bar 401 according tothe second embodiment of the present invention is positioned on thefirst rack bar support 303 for connecting frames (not shown) configuringpillars of the cassette in an X axis direction. The first rack bar 401is then extended toward a Y axis direction, and accordingly a rear endthereof is installed at a rear end of the cassette and fixed to thesecond rack bar support 305 which is installed in a Z axis direction.The first rack bar 401 has a certain height and is formed on the firstrack bar support 303 with the same interval. Hence, the first rack bars401 have spaces formed therebetween. Fingers of the robot arm are thuslyintroduced into and drawn out through the spaces to load or unload thesubstrate.

Each first rack bar 401, on the other hand, is connected to each secondrack bar 402 by a connection member. When the second rack bar 402 isfolded, the second rack bar 402 contacts a rear surface of the firstrack bar 401. When the second rack bar 402 is unfolded, the second rackbar 402 is positioned at a space between the first rack bars 401. Hence,the contact area between the rack bars and the substrate loaded on rackbars can be widened.

Therefore, the second rack bar 402 is extended toward a Y axis directionto be connected to the first rack bar 401 by using a connection membersuch as hinges which can be folded or unfolded.

The second rack bar 402 is formed to have a length shorter than that ofthe first rack bar 401, so that the second rack bar 402 is connected tothe first rack bar 401 so as to be folded or unfolded. That is, when thesecond rack bar 402 is unfolded, the first rack bar support 303 acts asan obstacle. Accordingly, in order to unfold the second rack bar 402without any interruption by the first rack bar support 303, the frontend of the second rack bar 402 is positioned with a gap from the firstrack bar support 303.

Furthermore, the rear end of the second rack bar 402 is also positionedwith a gap from the second rack bar support 305, thereby being folded orunfolded without any interruption by the second rack bar support 305.

The first rack bar 401 and the second rack bar 402 are connected totheir rear surfaces by the connection member 403 such as hinges, andaccordingly the second rack bar 402 can be folded. The second rack bar402 is unfolded and takes up a space formed between two first rack bars401 adjacent to each other. According to this, the contact area betweenthe substrate 301 and the rack bars can be widened. As a result, theload of the substrate can effectively be dispersed into the rack bars.

Moreover, when the second rack bar 402 is folded on the rear surface ofthe first rack bar 401, fingers of the robot arm are introduced into anddrawn out through the spaces formed between first rack bars adjacent toeach other to thereby load and unload the substrate 301.

On the other hand, a driving unit for operating the second rack bar 402is installed at a rear end of the second rack bar 402.

An operation of the second rack bar 402 will now be explained withreference to FIG. 4B.

FIG. 4B shows a rear end of the second rack bar 402, the A region of theFIG. 4A, and particularly shows a state in which the second rack bar 402is folded on the rear surface of the first rack bar 401. A second rackbar driving axis 405 is installed at a rear end of the second rack bar401 in an X axis direction. The rear end of the second rack bar 402 isconnected to the driving axis 405 by means of a sub-driving axis 410.Accordingly, when the driving axis 405 reciprocates in an X axisdirection, the second rack bar 402 is unfolded or folded from the rearsurface of the first rack bar 401 to the side surface thereof by thesub-driving axis 410.

When the substrate is loaded on the rack bars by the robot arm, thesecond rack bar 402 is folded on the rear surface of the first rack bar401. Afterwards, after the robot arm is taken away, when a sealant onthe substrate is hardened, the second rack bar 402 is unfolded tothereby support the substrate together with the first rack bar 401.

Therefore, when the sealant is hardened, the substrate is supported bythe first and second rack bars, and accordingly the supported areathereof is widened. As a result, the load of the substrate is uniformlydispersed into the first and second rack bars, which results in reducinga defect caused by a column spacer being pressed by the load of thesubstrate to thusly generate stain.

FIG. 4C is a front sectional view showing a state in which the first andsecond rack bars are unfolded, and FIG. 4D is a front sectional viewshowing a state in which the first and second rack bars are folded.

Referring to FIG. 5, on the other side, a substrate fixing unit 501which is formed of such rubber for preventing the substrate to be loadedfrom being slid is equipped on front and rear ends of the first rack bar401 and the second rack bar 402. As a result, the substrate can beprevented from being slid during the seal hardening process.

As described above with reference to the embodiments, in the sealhardening furnace according to the present invention, the rack barsbeing contact with the substrate are provided with the air dischargeopenings, or folding-type rack bars are provided, thereby preventingstain from being generated on the liquid crystal display penal whileperforming a sealant hardening process which is performed in a heatedstate.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalence of such metes and bounds are therefore intendedto be embraced by the appended claims.

1. A seal hardening furnace comprising: a cassette including rack barsfor supporting a substrate and having discharge openings therein, afirst rack bar support for supporting the rack bars in an X axisdirection, an injecting passage connected to the first rack bar support,and a supply unit for supplying gas through the injecting passage; and aheating unit for heating the substrate.
 2. The seal hardening furnace ofclaim 1, wherein the cassette has a sufficient size to support a liquidcrystal display panel provided with a lower substrate and an uppersubstrate which are attached each other by seal lines.
 3. The sealhardening furnace of claim 1, wherein a plurality of the rack bars arealigned with the same interval.
 4. The seal hardening furnace of claim3, wherein a plurality of the discharge openings is uniformlydistributed in an upper surface of the rack bar.
 5. The seal hardeningfurnace of claim 1, further comprising a second rack bar support formedat a rear end of the rack bar for supporting the rack bar in a Z axisdirection.
 6. The seal hardening furnace of claim 1, wherein the firstrack bar support has a path therein for inducing the gas coming inthrough the injecting passage to the discharge openings.
 7. The sealhardening furnace of claim 1, further comprising a discharger fordischarging the gas filled in the seal hardening furnace.
 8. The sealhardening furnace of claim 1, wherein the cassette is formed of aplurality of layers which are divided by a plurality of the first rackbar supports aligned in an X axis direction.
 9. The seal hardeningfurnace of claim 8, wherein each layer of the cassette is provided witha plurality of rack bars aligned parallel to one another in a Y axisdirection.
 10. The seal hardening furnace of claim 9, wherein both endsof each rack bar is fixed to the first rack bar support and the secondrack bar support installed in a Z axis direction, respectively.
 11. Aseal hardening furnace comprising: a plurality of first rack bars forsupporting a substrate, the first rack bars parallel to one another; aplurality of second rack bars making pairs with the plurality of firstrack bars; a first rack bar support for supporting a front end of thefirst rack bars; and a second rack bar driving unit installed at a rearend of each of the second rack bars.
 12. The seal hardening furnace ofclaim 11, wherein the first and second rack bars are connected to eachother by a connection member.
 13. The seal hardening furnace of claim11, wherein the second rack bar driving unit comprises: a first drivingaxis connected to the rear end of the second rack bar, for folding orunfolding the second rack bar from the first rack bar by reciprocatingin an X axis direction; and a sub-driving axis for connecting the firstdriving axis and the second rack bar.
 14. The seal hardening furnace ofclaim 11, wherein when the second rack bar is unfolded, a contact areawhere the first and second rack bars contact the substrate is increased.15. The seal hardening furnace of claim 11, further comprising asubstrate fixing member installed at front ends and rear ends of thefirst and second rack bars for preventing a substrate disposed on thefirst and second rack bars from sliding.
 16. A seal hardening furnacecomprising: a cassette including rack bars, a rack bar support disposedat an end of the rack bars that supports the rack bars, and a secondarysupport mechanism that increases uniformity of support for substratesdisposed on the rack bars beyond support provided by the rack barsalone; and a heating unit for heating the substrate.
 17. The sealhardening furnace of claim 16, wherein the secondary support mechanismcomprises a gas discharge mechanism that provides gaseous support forthe substrate such that the substrate is disposed over the rack barswithout contacting the rack bars.
 18. The seal hardening furnace ofclaim 17, wherein the gas discharge mechanism comprises dischargeopenings uniformly distributed on an upper surface of the rack bar. 19.The seal hardening furnace of claim 18, wherein the gas dischargemechanism comprises a conduit through the rack bar support through whichgas is introduced to the discharge openings.
 20. The seal hardeningfurnace of claim 17, further comprising a discharger for discharging gasfilled in the seal hardening furnace.
 21. The seal hardening furnace ofclaim 16, wherein the secondary support mechanism comprises a secondaryrack bar movably connected to each of the rack bars such that a contactarea of a surface of the rack bars and the substrate is extended whenthe secondary rack bar is in an extended position.
 22. The sealhardening furnace of claim 21, wherein the rack bars and secondary rackbars are connected to each other at multiple locations along a length ofthe rack bars and secondary rack bars.
 23. The seal hardening furnace ofclaim 21, wherein the secondary support mechanism further comprises asecondary rack bar driving unit that moves each secondary rack bar intoa compact position in which the secondary rack bar does not support thesubstrate and the extended position in which the secondary rack barsupports the substrate.
 24. The seal hardening furnace of claim 23,wherein the secondary rack bar driving unit is disposed at an end of thesecondary rack bars.
 25. The seal hardening furnace of claim 21, furthercomprising a substrate fixing member disposed at at least one ofopposing ends of the rack bars or at opposing ends of the secondary rackbars, the substrate fixing member preventing a substrate disposed on therack bars or secondary rack bars from sliding.
 26. The seal hardeningfurnace of claim 21, wherein when the secondary rack bars are in acompact position, the secondary rack bars contact a rear surface of therack bars.
 27. The seal hardening furnace of claim 21, wherein the rackbar support has a uniform width, and the secondary rack bars have alength shorter than a length of the rack bars such that when thesecondary rack bars are in the extended position, the secondary rackbars do not contact the rack bar support.